Long-Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
As shown in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of for example, 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RBs) where a resource block corresponds to one slot (0.5 ms) in the time domain and twelve contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a terminal device is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) indicated by the physical CFI channel (PCHICH) transmitted in the first symbol of the control region. The control region also contains physical downlink control channels (PDCCH) and possibly also physical Hybrid Automatic Repeat Request (HARQ) indication channels (PHICH) carrying acknowledgements for the uplink transmission.
The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
In dual connectivity (DC) a terminal device can be served by two nodes called master eNB (MeNB) and secondary eNB (SeNB). The terminal device is configured with a primary component carrier (PCC) from both the MeNB and the SeNB. The primary cell from the MeNB and the SeNB are called a PCell and PSCell respectively. The PCell and PSCell typically operate the terminal device independently. The terminal device is also configured with one or more Secondary Component Carriers (SCCs) from each of MeNB and SeNB. The corresponding secondary serving cells served by MeNB and SeNB are called SCells. The terminal device in DC typically has separate transmitting and receiving functions for each of the connections with MeNB and SeNB. This allows the MeNB and SeNB to independently configure the terminal device with one or more procedures e.g. radio link monitoring (RLM), discontinuous reception cycle etc. on their PCell and PSCell respectively.